Apparatus for and method of controlling feed of grinding media to a grinding mill

ABSTRACT

An apparatus for and a method of controlling the feed rate of grinding media to a grinding mill of the type which operates at a power draft in the vicinity of the critical peak power draft of the grinding mill. In the described embodiment, the ore grinding media feed rate to a secondary autogenous grinding mill is controlled. In the described embodiment the ore grinding media feed rate is normally automatically controlled in response to the electrical power demand of the grinding mill by control means whose function is to maintain the power draft of the mill at a predetermined set point. At suitable time intervals, such as once every sixty minutes, the normal automatic feed control for the ore grinding media is interrupted or deactivated and an override feed control for the ore grinding media is substituted in place of the normal feed control. The override feed control acts to substantially decrease the rate of feed of the ore grinding media to the mill sufficiently to cause a measurable effect on the power draft of the mill. During the period when the override control is in effect, the feed of ore grinding media to the mill may even be stopped completely. A sensing device responsive to power increase or decrease detects whether the power input to the mill decreases or increases when the override feed control is in control, thereby indicating whether the mill is loaded below or above the grinding media charge or loading corresponding to the critical peak power draft of the mill. If the sensing device detects a decrease in power input to the mill during the override period, the override ore grinding media feed control is disconnected or otherwise deactivated and the normal ore grinding media feed control is reconnected or otherwise reactivated. If the sensing device detects an increase in power input to the mill during the override period, the override feed control remains connected to provide a decreased rate of feed of ore grinding media, which may even include a complete stoppage of feed of ore grinding media, until the sensing device detects a decrease in power input to the mill, at which time the override ore grinding media feed control is disconnected and the normal ore grinding media feed control is reconnected.

United States Patent Bond [ Nov. 20, 1973 APPARATUS FOR AND METHOD OFCONTROLLING FEED OF GRINDING MEDIA TO A GRINDING MILL [75] Inventor:Fred C. Bond, Tucson, Ariz.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee, Wis.

[22] Filed: Feb. 25, 1972 [21] Appl. No.: 229,454

[52] US. Cl. 241/30, 241/34 [51] Int. Cl. B02c 25/00 [58] Field ofSearch 241/30, 33, 34, 35,

[56] References Cited UNITED STATES PATENTS 2,952,414 9/1960 Williamson..L 241/30 2,336,434 12/1943 Wurzbach 241/30 Primary ExaminerGranvilleY. Custer, Jr. Attorney-Robert C. Sullivan et al.

[5 7 ABSTRACT An apparatus for and a method of controlling the feed rateof grinding media to a grinding mill of the type which operates at apower draft in the vicinity of the critical peak power draft of thegrinding mill. In the described embodiment, the ore grinding media feedrate to a secondary autogenous grinding mill is controlled. In thedescribed embodiment the ore grinding media feed rate is normallyautomatically controlled in response to the electrical power demand ofthe grinding mill by control means whose function is to MEDIA NORMALFEED CONTROL FOR GRINDING MEDIA povvexz DECREASE SIGNAL 54 OVERRIDE FEEDCONTROL FOR GRINDING MEDIA maintain the power draft of the mill at apredetermined set point. At suitable time intervals, such as once everysixty minutes, the normal automatic feed control for the ore grindingmedia is interrupted or deactivated and an override feed control for theore grinding media is substituted in place of the normal feed control.The override feed control acts to substantially decrease the rate offeed of the ore grinding media to the mill sufficiently to cause ameasurable effect on the power draft of the mill. During the period whenthe override control is in effect, the feed of ore grinding media to themill may even be stopped completely. A sensing device responsive topower increase or decrease detects whether the power input to the milldecreases or increases when the override feed control is in control,thereby indicating whether the mill is loaded below or above thegrinding media charge or loading corresponding to the critical peakpower draft of the mill. If the sensing device detects a decrease inpower input to the mill during the override period, the override oregrinding media feed control is disconnected or otherwise deactivated andthe normal ore grinding media feed control is reconnected or otherwisereactivated. If the sensing device detects an increase in power input tothe mill during the override period, the override feed control remainsconnected to provide a decreased rate of feed of ore grinding media,which may even include a complete stoppage of feed of ore grindingmedia, until the sensing device detects a decrease in power input to themill, at which time the override ore grinding media feed control isdisconnected and the normal ore grinding media feed control isreconnected.

8 Claims, 5 Drawing Figures FINE ORE OR THE LIKE BEING PROCESSEDELECTRIC POWER SUPPLY 5 POWER INCREASE 7 OR DECREASE SENSING DEVICEPATENTED NOV 2 01975 CRITICAL PEAK POWER DRAFT MILL CHARGE (ORE GRINDING'MEDIA) ZOEIQEDMZOU NEE/On U NEIum m CONVENTIONAL CRUSHING PLANT I D F 60% W I E mm I N mD IWD I C G 3 N. E R S 0 R V O L D wa H M E 3:4 W W FSECONDARY AUTO GENOUS GRINDING MILL APPARATUS FOR AND METHOD OFCONTROLLING FEED OF GRINDING MEDIA TO A GRINDING MILL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to anapparatus for and a method of grinding mineral ore and more particularlyto an apparatus for and a method of controlling the feed of grindingmedia for use with grinding mills of the type which operate at a powerdraft in the vicinity of the critical peak power draft of the grindingmill.

The invention is of particular utility in controlling the feed of theore grinding media to a secondary autogenous grinding mill of the typein which the ore grinding media, which may be, for example, ore having amesh of from 3 inches to +1 inches, is fed to the secondary autogenousmill to serve as a grinding media for the fine feed ore which may, forexample, be of 8 mesh. However, in its broader sense, the invention isapplicable to controlling the input feed of grinding media to anygrinding mill of a type which is capable of operating in the region ofits critical peak power draft beyond which an increase in the totalgrinding media in the mill will cause the power draft of the mill todiminish. Thus, if applied to a ball or tube mill, the feed of metal ornonmetal grinding balls or other shapes used for grinding would becontrolled by the present invention; and if applied to a primaryautogenous or to a semi-autogenous grinding mill, the entire feed ofmaterial to the mill would be controlled. The term grinding media asused in the specification and claims is intended to be generic to theore grinding media which is fed to a secondary autogenous grinding mill,to the metal or nonmetal balls or other shapes used for grinding in aball or tube mill, and to the entire feed of material to a primaryautogenous or to a semiautogenous grinding mill.

'2. Description of the Prior Art Grinding mills of the secondaryautogenous type using ore or rock as the grinding media (hereinafterreferred to as ore grinding media) are usually provided with anautomatic control on the rate of the ore grinding media feed which isvaried automatically so as to seek to maintain a constant set powerdraft by the grinding mill. In addition to the ore grinding media whichserves as the grinding medium, the mill is also supplied with what isknown in the art as fine feed ore which may, for example, be the outputproduct of a rod mill having, for example, a -8 mesh grind. The feedrate of the fine feed ore to the grinding mil] in the prior art and alsoin the environment of the present invention is responsive to controlmeans which are separate and distinct from the feed control means forthe ore grinding media, the control means for the fine feed ore formingno part of the present invention. Furthermore, variations in the rate offeed of the fine feed ore affect the power draft of the grinding millonly slightly.

In the prior art secondary autogenous mill control arrangements theinput power set point is commonly set appreciably less than the criticalpeak power which the mill would draw with a full charge of ore grindingmedia. For example, in the prior art, the power set point may be set atperhaps 90 percent or 85 percent of the critical peak power. If the millpower draft (demand) rises above the set point, the ore grinding mediafeed in the prior art secondary autogenous grinding mill control systemsis automatically reduced, and if the power draft by the mill falls belowthe set point, the ore grinding media feed rate is automaticallyincreased, so as to seek to maintain the average power drawn by the millat the set point.

In the prior art control systems for secondary autogenous grindingmills, the set point must be appreciably lower than the critical peakpower of the grinding mill in order to safely avoid overshooting" whichwould increase the amount of the total ore grinding media charge in themill beyond that requiring the critical peak power draft or demand. Ifthe total ore grinding media charge (i.e., the total amount of oregrinding media present in the mill at a given moment) exceeds thatrequiring the critical peak power, the power draft will decrease (seeFIG. 2 of the drawing). The prior art control systems for controllingthe feed of ore grinding media in a secondary autogenous grinding millof which I am aware, are unable to discriminate as to whether the millis operating on the right or wrong side of the critical peak power drafton the curve of power draft vs. mill charge of ore grinding media. Oncethe mill power draft is beyond the critical peak power point beyondwhich the input power to the mill decreases with increase in oregrinding media charge, most prior art mill feed control systems areineffective since a decrease in power draft calls for an increase in oregrinding media feed rate which still further overloads the mill beyondthe grinding capacity of the mill, causing a still further decrease ininput power to the mill. When this happens, the only alternative in theprior art feed control system is for the operator to manually cut offthe ore grinding media feed and permit the excess charge in the mill tobe ground out before automatic feed control is resumed. This can resultin serious loss in grinding capacity of the mill.

STATEMENT OF THE INVENTION Accordingly, it is an object of the presentinvention to provide an apparatus for and a method of controlling thefeed of grinding media to a grinding mill which is particularly adaptedfor use with grinding mills of the type which are capable of beingloaded to a power draft set point in the vicinity of, but less than, thecritical peak power draft of the mill and in accordance with which thegrinding mill may be operated closer to the critical peak power draftpoint then in the prior art apparatus and methods.

It is a further object of the invention to provide an apparatus for, andmethod of controlling the feed of grinding media to a grinding mill inaccordance with which a given grinding mill may operate closer to itspeak grinding capacity than in prior art apparatus and methods.

It is another object of the invention to provide an apparatus for and amethod of controlling the feed of grinding media to a grinding millwhich is particularly suitable for, but not restricted to, use inconnection with secondary autogenous grinding mills and which permitsthe secondary autogenous grinding mill to operate closer to its criticalpeak power draft than in the prior art, thereby increasing the overallcapacity of the given grinding mill.

It is a further object of the invention to provide an apparatus for anda method of operating a grinding mill at a power draft only slightlybelow the critical peak power draft of the given mill, while at the sametime preventing the buildup of an excess charge of grinding media in themill.

It is still another object of the invention to provide an apparatus forand method of controlling the feed of grinding media which candiscriminate as to whether the mill is operating on the right or on thewrong side of the critical peak power draft on the curve of power draftvs. charge of grinding media for the mill and which can make appropriatecorrections in the loading of grinding media if it is determined thatthe mill is operating on the wrong side of the critical peak powerdraft.

In achievement of these objectives, there is provided in accordance withan embodiment of this invention an apparatus for and a method ofcontrolling the feed rate of grinding media to a grinding mill of thetype which operates at a power draft in the vicinity of the criticalpeak power draft of the grinding mill. In the described embodiment, theore grinding media feed rate to a secondary autogenous grinding mill iscontrolled. In the described embodiment, the ore grinding media feedrate is normally automatically controlled in response to the electricalpower demand of the grinding mill by control means whose function is tomaintain the power draft of the mill at a predetermined set point. Atsuitable time intervals, such as once every 60 minutes, the normalautomatic feed control of the ore grinding media is interrupted ordeactivated and an override feed control for the ore grinding media issubstituted in place of the normal feed control. The override feedcontrol acts to substantially decrease the rate of feed of the oregrinding media to the mill sufficiently to cause a'measurable effect onthe power draft of the mill. During the period when the override controlis in effect, the feed of ore grinding media to the mill may even bestopped completely. A sensing device responsive to power increase ordecrease detects whether the power input to the mill decreases orincreases when the override feed control is in control, therebyindicating whether the mill is loaded below or above the ore grindingmedia charge corresponding to the critical peak power draft of the mill.If the sensing device detects a decrease in power input to the millduring the override period, the override ore grinding media feed controlis disconnected or otherwise deactivated and the normal ore grindingmedia feed control is reconnected or otherwise reactivated. If thesensing device detects an increase in power input to the mill during theoverride period, the override feed control remains connected to providea decreased rate of feed of ore grinding media (which may even include acomplete stoppage of feed of ore grinding media), until the sensingdevide detects a decrease in power input to the mill, at which time theoverride ore grinding media feed control is disconnected and the normalore grinding'media feed control is reconnected.

BRIEF DESCRIPTION OF vTI-IE DRAWINGS Further objects and advantages ofthe invention will become apparant from the following description takenin conjunction with the accompanying drawing in which:

FIG. I is a schematic diagram illustrating the relationship of thevarious parts of the apparatus of the invention;

FIG. 1A is a schematic diagram showing the components of a powerincrease or decrease sensing device which may be used;

FIG. 2 is a graph showing the relationship between electrical powerconsumption by the grinding mill and the charge of ore grinding media inthe mill;

FIG. 3 is a schematic diagram showing the material which is fed into asecondary autogenous grinding mill; and

FIG. 4 is a schematic diagram of a modified control arrangement inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing andmore particularly to FIG. 1, there is shown a grinding mill, generallyindicated at 10, of the secondary autogenous type. Mill 10 receives afine feed ore 12 which may be, for example, 8 mesh, from I a hoppergenerally indicated at 14 which dispenses fine feed ore 12 in the formof a slurry into inlet end 16 of the grinding mill. As best seen in theschematic diagram of FIG. 3, fine feed ore 12 may be the output of a rodmill which in turn is fed at the input end thereof by the -%r inch mesh,for example, output of a conventional crushing plant. The rate of feedof the fine feed ore 12 to mill 10 is separately controlled by meanswhich form no part of this invention.

The ore grinding media which serves as the grinding media in mill 10 isindicated at 18 and is dispensed from a hopper 20 onto a moving conveyorbelt 22 which delivers the ore grinding media to the input spout or thelike 16 of the grinding mill l0, ore grinding media 18 becoming mixedwith fine feed ore l2 inside the grinding mill. As seen in the schematicdiagram of FIG. 3, the ore grinding media may be of a mesh size 3 to+1-% inches and is derived from a stockpile which is supplied by theconventional crushing plant shown in FIG. 3. In other words, the finefeed ore (for example, 8 mesh) and the ore grinding media 18 (forexample, 3 inches to +1-% inches mesh) are derived from the same sourceof ore. The ore grinding media 18 serves as the grinding medium for thefine feed ore l2 and eventually is reduced in size itself within mill 10to become part of the output product of grinding mill 10 which isdischarged continuously through discharge outlet 17 as the mill rotates.

The feed device 24 controls the position of a variable adjustment gate26 on hopper 20 whereby to control the rate of feed of the ore grindingmedia 18 onto the moving conveyor belt 22. It is obvious that the rateof feed of the ore grinding media could also be controlled bycontrolling the rate of movement of conveyor belt 22.

An electric drive motor 28 rotatably drives the grinding mill 10 througha pinion 30 which engages a ring gear 32 on the outer periphery ofgrinding mill 10, as is conventional in the art.

A kilowatt meter or kilowatt-hour meter 34 measures the electrical powerinput to drive motor 28.

Referring now to FIG. 2 which shows the relationship between the chargeof ore grinding media in grinding mill l0 and the electrical powerconsumption of grinding mill 10 as measured on the kilowatt meter 34, itwill be noted that as the charge of the ore grinding media 18 in themill 10 increases, the electrical power consumption of the grinding mill10, as measured by the kilowatt meter 34, also increases along theportion of the curve indicated at A-B in FIG. 2. Point B of the curve inFIG. 2 represents the critical peak power consumption by grinding mill10. If the charge of ore grinding media in the mill is increased beyondthe value D which corresponds to the critical peak power point B on thecurve of FIG. 2, the power drawn by the mill will decrease even thoughthe total charge of ore grinding media in the mill is increased. Thusthe portion B-C of the power curve of FIG. 2 shows a decline in powerdrawn by the mill even though the abscissa of the graph (total charge ofore grinding media in the mill) is greater in portion 8- C of the curvethan in the portion A-B of the curve.

When the power draft of the mill passes beyond the critical peak powerpoint B, the mill becomes ineffective as a grinding mill and insubstance becomes a rotating flywheel which is no longer effectivelygrinding the charge in the mill.

As seen in the diagrammatic view in FIG. 1, the normal feed controlmeans for the ore grinding media 18 is indicated at 36. The normal feedcontrol 36 controls the rate of ore grinding media feed as a function ofthe power demand by grinding mill 10. Normal feed control 36 isconnected by the conductor means 38 to kilowatt meter or kilowatt-hourmeter 34 and by conductor means 40 to one terminal 48 of a two-wayswitch generally indicated at 42. Two-way switch 42 includes a movableswitch contact 44 which normally engages switch terminal 48. An overridefeed control means arranged to either decrease the rate of feed of oregrinding media 18 as compared to the rate of feed of ore grinding mediaprovided by the normal feed control 36, or, alternatively, to entirelystop the feed of ore grinding media 18 to mill 10, is indicated at 52.Override feed control 52 is connected by conductor means 54 to terminal46 of two-way switch 42. Timer 50 is mechanically connected to movableswitch contact 44 in such manner that at suitable predetermined timeintervals, such as, for example, once every sixty minutes, time 50 willmove movable switch contact 44 out of engagement with fixed terminal 48in which movable contact 44 is connected to the normal feed control 36,and into contact with the fixed terminal 46 in which movable contact 44is connected to override feed control 52. The fixed end of movablecontact 44 is connected by conductor means 56 to feed device 24. It willthus be seen that when the movable contact 44 of switch 42 is inengagement with the fixed terminal 48, feed device 24 is electricallyconnected to the normal feed control 36; and when movable contact 44 ofswitch 42 is in engagement with fixed terminal 46, feed device 24 isconnected to override feed control 52.

A power increase or decrease sensing device 57 which is capable ofsensing either an increase or a decrease in power input to grinding millis connected by electrical conductor means 58 to the kilowatt meter 34in series with a normally open contact 60 which is closed by timer 50when movable contact 44 is moved into engagement with fixed terminal 46.Conductor means 64 transmits a power decrease signal (assuming thatthere is a power decrease signal) from sensing device 57 to relay 66which when energized or triggered by the power decrease signal, returnsmovable contact 44 into engagement with fixed terminal 48 to therebydisconnect override feed control 52 from feed device 24 and to reconnectnormal feed control 36 to feed device 24. Relay 66 when energized ortriggered by the power decrease signal also opens contact and thusdisconnects the sensing device 57 from kilowatt meter 34. Override feedcontrol 52 remains connected in controlling relation to feed device 24until relay 66 is triggered by a power decrease signal as justdescribed.

Instead of utilizing the power decrease output signal from device 57 totrigger relay 66 in response to a decrease in power consumption by mill10 during the override period, as just described, a power increasesignal indicative of an increase in power consumption by mill [0 duringthe override period may be transmitted by conductor means 64 to relay66. In this latter case, as long as a power increase signal istransmitted to relay 66 from device 57, relay 66 will remain in acondition in which override feed control 52 is connected to feed device24 and normal feed control 36 is disconnected. However, in the absenceof a power increase signal transmitted from device 57 to relay 66, relay66 will be actuated to disconnect override feed control 52 and toreconnect normal feed control 36 to feed device 24.

Normal feed control 36 may be an analog controller such as Foxboro Model67HSTGZ-E, made by Foxboro Co., Foxoboro, Mass., as described in FoxboroBulletin No. GS2A- 4D1B, July, 1966, or instead may be a digitalcomputor, such as Model TCT88, made by Foxboro Co., of Foxboro, Mass,described in Foxboro Bulletin L-l9, November, 1965.

The override feed control 52 may be an electrical power supply and apotentiometer connected to the power supply and adjusted to provide asignal current through conductor means 54, 56 which causes the feeddevice 24 to feed the ore grinding media at a desired reduced rate. Ifit is desired to have a progressive decrease in feed rate provided byoverride control 52, such progressive decrease may be provided by meansof a suitable timer or clock control mechanism for providing aprogressive variable adjustment of the potentiometer.

The power increase or decrease sensing device 57 may comprise theelements schematically shown in FIG. 1A. As shown, device 57 includes aSample and Hold Amplifier No. 1 connected to kilowatt meter 34 toreceive a signal respresentative of the electrical power demand ofgrinding mill 10 at a time T corresponding substantially to the momentwhen the override perior first begins and before the override feedcontrol 52 has had any effect on the electrical power demand by mill 10.Device 57 also includes a Sample and Hold Amplifier No. 2 connected tokilowatt meter 34 to receive a signal representative of the electricalpower demand of grinding mill 10 at a time T sufficiently after thecommencement of the override period for the override control to have hada measureable effect on the electrical power demand by mill 10. Thesignals which have been sampled by the respective Sample and HoldAmplifiers No. l and No. 2 are fed into a comparator 63 which comparesthe signals from amplifiers No. 1 and No. 2 to determine whether thepower demand by mill 10 at time T is less than or greater than the powerdemand by mill 10 at time T, Preferably only the comparator outputsignal corresponding to one of these conditions just mentioned, such asthe less than signal corresponding to a decrease in power consumption bymill 10 at time T is fed over conductor means 64 to the utilizationdevice such as relay 66.

The Sample and Hold Amplifiers No. 1 and No. 2 of FIG. 1A may be thesample-hold module Model F8201 or F8202, manufactured by lntronics,Incorporated, 57 Chapel St., Newton, Mass. 02158, as described in the1971-1972 Electronic Engineers Master Catalog, Vol. 2, page 1288,published by United Technical Publications, Inc., 645 Stewart Ave.,Garden City, N. Y. 11530.

The comparator 63 of FIG. 1A may be Voltsensors Model 552, precisionsolid state voltage comparator, manufactured by California ElectronicsMfg. Co., Inc., P.O. Box 555, Alamo, Calif. 94507, as described in theaforementioned 1971-1972 Electronic Engineers Master Catalog, Vol. 2,pages 1315. 7

While the normal feed control 36 and the override feed control 52 haveben shown and described as controlling the feed device 24 which controlsthe position of gate 26, it is obvious that the controls 36 and 52 couldinstead be used for controlling the speed of the belt conveyor 22,thereby controlling the rate of feed of the ore grinding media to mill10.

Summary of Operation ln operating in accordance with the apparatus andmethod of the embodiment of the invention, shown in FIG. 1, the feeddevice 24 which actuates the gate 26 to control dispensing of the oregrinding media 18 onto conveyor belt 22, is normally automaticallycontrolled in accordance with the power draft of drive motor 28, asindicated on kilowatt meter 34. Thus, in normal operation, feed device24 is electrically connected to normal feed control 36, which isresponsive to the reading of kilowatt meter 34, through the connectionof movable contact 44 of two-way switch 42 to terminal 48 whereby thenormal feed control device 36 is connected through switch 42 to feeddevice 24. When the kilowatt meter 34 indicates that the power drawn bythe mill is below the set point S (FIG. 2) the normal feed control 36acts to cause the feed device 24 to increase the feed rate of the oregrinding media 18; and, conversely, when kilowatt meter 34 indicatesthat the mill is drawing power above the set point S, the normal feedcontrol 36 acts to cause feed device 24 to diminish the rate of feed ofore grinding media 18 to conveyor 22, thus tending to reduce the powerdraft of the grinding mill back to the set point S.

At suitable time intervals, say once in every sixty minutes, forexample, timer 50 moves movable contact 44 of two-way switch 42 intoengagement with fixed terminal 46 to thereby disconnect normal feedcontrol 36 from feed device 24 and to connect the override feed control52 to feed device 24. At the same time, timer 50 acts to close contact60 to thereby connect power increase or decrease sensing device 57 tokilowatt meter 34.

The override feed control 52 is so adjusted as to reduce the rate offeed of ore grinding media by feeding device 24 to mill l considerablybelow the level at which it operated when subject to the normal feedcontrol 36. Override feed control 52 may even be so adjusted as to causethe complete stopping of the feed of ore grinding media to mill 10.

Since the reduction in rate of feed of ore grinding media provided bythe override feed control 52 is such as to cause a measurable effect onelectrical power consumption by mill 10 within a few minutes, powerincrease or decrease sensing device 57 will detect a change in powerconsumption within a short time after the override feed control 52 isconnected in controlling relation to feed device 24.

If, during the period that the override feed control 52 is connected tofeed device 24, the mill 10 is operating on portion AB of the powerinput curve, it can be seen from an inspection of FIG. 2 that a decreasein the total charge of ore grinding media in the mill as caused by theeffect of the override feed control 52 with its lesser rate of feed oralternatively, its entire stoppage of feed, will cause a decrease in thepower consumption by the grinding mill. Hence, if during the period whenthe override feed control 52 is connected to feed device 24, the mill isoperating on portion AB of the curve, the power increase or decreasesensing device 57 will sense a decrease in power consumption due to thedecrease in mill loading caused by override feed control 52. In thiscase, a power decrease signal will be transmitted by device 57 overconductor means 64 to relay 66 causing relay 66 to return movablecontact 44 into engagement with fixed terminal 48, thereby disconnectingthe override feed control 52 from feed device 24 and reconnecting normalfeed control 36 to feed device 24. When energized or triggered by thepower decrease signal, relay 66 will also open contact 60, therebydisconnecting sensing device 57 from kilowatt meter 34. The normal feedcontrol 36 when reconnected will act to increase the feed rate of oregrinding media to mill 10 to again reach set point S on the curve ofFIG. 2. The normal feed control 36 will remain connected to feed device24 until the next override perior occurs as determined by timer 50.

However, if, during the period when override feed control 52 isconnected to feed device 24, the power increase or decrease sensingdevice 57 detects an increase in power consumption despite the decreasein mill charge of ore grinding media caused by the operation of overridefeed control 52, this will indicate that the grinding mill is operatingon portion B-C of the power curve of FIG. 2 and thus is operating beyondthe critical peak power draft of the grinding mill. If this conditionprevails, a power decrease signal will not be transmitted over conductormeans 64 to relay 66 and relay 66 will remain in a position in whichoverride feed control 52 remains connected to feed device 24 to continuethe reduced rate of feed of ore grinding media to mill 10. Override feedcontrol 52 may be of a type which if it continues to remain connected tofeed device 24 causes a progressive decrease in the rate of ore mediafed by feed device 24. Alternatively, as previously mentioned, overridefeed control 52 when in control may entirely stop the feed of oregrinding media to mill 10.

The decreased rate of feed by feed device 24 which may include acomplete stoppage of feed of ore grinding media, under the control ofthe override feed control 52, as just described, will continue until thetotal charge of ore grinding media in the mill decreases to a point lessthan loading point D of FIG. 2 (loading point D corresponds to thecritical maximum power draft B), and the mill begins to again operate onthe portion AB of the power draft curve below the critical maximum powerdraft. When this point in the mill loading is reached, the powerincrease or decrease sensing device 57 will then begin to sense adecrease in power input to grinding mill 10 with further decrease intotal charge of ore grinding media in the mill. The power increase ordecrease sensing device 57 will then transmit a power decrease signalover conductor means 64 to relay 66, as previously described, causingrelay 66 to move contact 44 of switch 42 out of engagement with terminal46 and into engagement with terminal 48, thereby reconnecting normalfeed control 36 to feed device 24. The movable contact 44 will thenremain in engagement with terminal 48 as just described, until the nextoperation of timer 50 moves contact 44 back into engagement withterminal 46, as previously described. The power decrease signal willalso cause relay 66 to open contact 60, as previously explained.

Modified Control Arrangement of FIG. 4

Referring now to FIG. 4, there is shown a modified control arrangementfor controlling the feed of ore grinding media to the secondaryautogenous type grinding mill generally indicated at 100. Mill 100receives five feed ore 112 from a hopper generally indicated at 114 inthe same manner as described in connection with the embodiment ofFIG. 1. The rate of feed of the fine ore 112 to mill 100 is separatelycontrolled by means which form no part of this invention.

The ore grinding media 118 which serves as the grinding media in mill100 issimilar to that described in connection with the embodiment ofFIG. 1 and is-dispensed from a hopper 120 onto a moving conveyor belt122. In the embodiment illustrated in FIG. 4, the rate of feed of theore grinding media to the grinding mill 100 is controlled by controllingthe speed of motor 127 which drives the movable conveyor belt 122.However, the rate of feed of the ore grinding media 118 to mill 100 mayalso be controlled by controllingafeed device 24 which controls theposition of a variable adjustment gate 26 in the same manner asdescribed in connection with the embodiment of FIG. 1.

An electric drive motor 128 rotatably drives the grinding mill 100through a pinion 130 which engages a ring gear 132 on the outerperiphery of grinding mill 100, as is conventional in the art.

A kilowatt meter or kilowatt-hour meter 134 measures the power input tomill drive motor 128.

As seen in the diagramatic view of FIG. 4, a normal feed control 136 forthe ore grinding-media 118 is provided and is similar to the normal feedcontrol 36 of the embodiment of FIG. 1. Normal feed control 136 isconnected by conductor means 138 to kilowatt meter 134 and by conductormeans 140 to terminal 148 of a single pole switch generally indicated at142. The normal feed control 136 controls the rate of feed of oregrinding media to grinding mill 100 as a function of the power demand bygrinding mill 100. Switch 142 includes a movable switch contact 144which normally engages switch terminal 148. Movable switch contact 144is connected by conductor means 156 to motor 127 which drives conveyorbelt 122 which delivers ore grinding media 118 to input spout 116 ofgrinding mill 100. A timer 150 is mechanically connected to movableswitch contact 144 in such manner that at suitable predetermined timeintervals, such as, for example, once every 60 minutes, timer 150 movesswitchcontact 144 out of contact with terminal 148 and thus disconnectsnormal feed control 136 from its electrical connection to belt drivemotor 127, thereby causing motor 127 and hence conveyor belt 122 to stopoperating during the period when normal feed control 136 isdisconnected. When conveyor belt 122. is not operating, as justdescribed, no ore grinding media 118 is delivered to mill 100. Thedisconnection of normal feed control 136 from its electrical'connectionto belt drive motor 127, as just described, in effect overrides thenormal feed control 136 in the sense used in the claims.

A power increase or decrease sensing device 157 which is capable ofsensing either an increase or a decrease in power input to grinding mill100, and which may be similar to power increase or decrease sensingdevice 57 of FIG. 1, is connected by electrical conductor means 158 tokilowatt meter 134 in series with a normally open contact 160 which. isclosed by timer 1 150 when movable contact 144 is moved out ofengagement with terminal 148. Conductor means 164 transmits a powerdecrease output signal from the power increase or decrease sensingdevice 157 to a relay 166 when device 157 senses a decrease in the powerconsumption by grinding mill in the same manner as described in.connection with the embodiment of FIG. 1. Relay 166 isenergized ortriggered by the power decrease signal from the power increase ordecrease sensing device 157. Energization of relay 166 by a powerdecrease signal causes relay 166 to return movable contact 144 intoengagement with terminal 148, thereby reconnecting the normal-feedcontrol 136 to the belt drive motor 127 and thereby causing the conveyorbelt 122; to resume movement to again deliver ore grinding media to mill100.

Summary of Operation of Embodiment of FIG. 4

In operating in accordance with the apparatus and method of theinvention as shown in the modified arrangement of FIG. 4, the belt drivemotor 127 which drives conveyor belt 122' and thus controls the rate ofdispensing of the ore grinding media 118 to grinding mill 100, isnormally automatically controlled as a function of the power draft bymilldrive motor 128, as indicated on kilowatt meter 134. Thus, in normaloperation the speed of the belt drive motor 127 is controlled by thenormal feed control 136, which is responsive to the reading of kilowattmeter 134, through the connection of the movable contact 144' of switch142 to the fixed terminal 148, whereby the normal feed control device136 is connected through switch 142 to belt drive motor 127. Whenkilowatt meter 134 indicates that the power drawn by the mill is belowthe set point 8 (FIG. 2), the normal feed control 136 acts to cause thebelt drive motor 127 to increase the feed rate of the ore grinding media118 to mill 100; and conversely, when kilowatt meter 134 indicates thatthe mill is drawing power above the set point S, the normal feed control136 acts to cause the belt drive motor 127 to diminish the rate of feedof ore grinding media 118 to grinding mill 100, thus tending to reducethe power draftby the grinding mill back to the set point S.

At suitable time intervals, say once in every 60 minutes, for example,timer moves movable contact 144 out of engagement with fixed terminal148, thereby disconnecting normal feed control 136 from belt drive motor127 thereby causing belt drive motor 127 to stop operating, stoppingconveyor belt 122, and thereby discontinuing the feed of ore grindingmedia 118 to grinding mill 100. At the same time, timer 150 acts toclose contact to thereby connect power increase or decrease sensingdevice 157 to kilowatt meter 134. A power decrease signal is transmittedby device 157 over conductor means 164 to relay 166 when device 157senses a decrease in the power draft of mill 100.

Since the complete stoppage in the'feed of ore grinding media 118 to thegrinding mill 100 due to the opening of switch 142 as just described,causes a measurable effect on the power consumption by mill 100 within afew minutes, power increase or decrease sensing device 157 will detect achange in power consumption by mill 100 within a short time after switch142 is opened.

If, during the period when switch 142 is open and the normal feedcontrol 136 is electrically disconnected from belt drive motor 127, withresultant stoppage of feed of ore grinding media to grinding mill 100,the mill 100 is operating on portion A-B of the power input curve ofFig. 2, it can be seen from an inspection of FIG. 2 that a decrease inthe total charge of ore grinding media in the mill will cause a decreasein the power consumption by the grinding mill. Hence, if during theoverride period when the belt drive motor 127 has stopped clue toopening of switch 142, the mill is operating on portion A-B of thecurve, the power increase or decrease sensing device 157 will sense adecrease in power consumption. In this case, an electrical signalindicative of the power decrease will be transmitted by device 157 overconductor means 164 to relay 166, causing relay 166 to return movablecontact 144 into engagement with terminal 148, thereby reconnecting thenormal feed control 136 to belt drive motor 127. When energized ortriggered by the power decrease signal, relay 166 will also open contact160, thereby disconnecting sensing device 157 from kilowatt meter 134.The normal feed control 136 when reconnected will reenergize belt drivemotor 127, causing conveyor belt 122 to again resume the feeding of oregrinding media to mill 10, to again reach set point S on the curve ofFIG. 2. The normal feed control 136 will remain connected to belt drivemotor 127 until the next override period occurs, as determined by timer150.

However, if, during the period when switch 142 is opened and conveyorbelt 122 is not moving, with resultant complete stoppage of feed ofgrinding media 1 18 to mill 100, the power increase or decrease sensingdevice 157 detects an increase in power consumption despite the decreasein mill charge caused by the disconnection of the normal feed control136, this will indicate that the grinding mill is operating on portionB-C of the power curve of FIG. 2, and thus is operating beyond thecritical peak power draft to the grinding mill. In this case, relay 166will not be triggered into operation, and switch 142 will remain open,with belt drive motor 127 remaining disconnected from feed control 136.

As the belt drive motor 127 continues to remain disconnected from thenormal feed control 136 and thus the complete stoppage of the feed ofore grinding media 118 to grinding mill 100 continues, the total chargeof ore grinding media 118 in the mill will eventually decrease to apoint less than the loading point D corresponding to the criticalmaximum power draft B and the mill will begin to again operate on theportion A-B of the power draft curve below the critical maximum powerdraft. When this point in the mill loading is reached, the powerincrease or decrease sensing device 157 will then begin to sense adecrease in power input to grinding mill 100 and will transmit-a powerdecrease signal over conductor means 164. to relay 166 causing relay 166to move contact 144 of switch 142 into engagement with terminal 148,thereby reconnecting normal feed control 136 to belt drive motor 127.Belt drive motor 127 will resume operation, thereby causing the feed orore grinding media 118 to mill 100 to resume. The power decrease signalwill also cause relay 166 to open contact 160 to thereby disconnect thepower increase or decrease sensing device 157 from kilowatt meter 134,as previously explained. Switch 142 will remain closed with movablecontact 144 in engagement with terminal 148, as just described, untilthe next operation of timer 150 reopens switch 142, as previouslydescribed.

It can be seen from the foregoing that there is provided in accordancewith the embodiments of the invention an apparatus and method whichprovide a check of the grinding mill operation at suitable intervals todetermine whether the grinding mill is operating on the portion A-B ofthe power draft curve or on the portion B-C of the power draft curve,and for making appropriate adjustments to the rate of ore grinding mediafed to the mill if the control determines that the mill is operating ata point on the power draft curve beyond the point of critical maximumpower draft. Because of the checking and appropriate grinding media feedadjustment made, if necessary, in accordance with the apparatus andmethod of the invention, the mill may be operated at a set point whichis closer to the point of critical maximum power draft than is the casein connection with the prior art apparatus and method. As previouslymentioned, it is common practice in accordance with the prior art toestablish the set point of the mill power draft at a point which issubstantially lower than the point of critical peak power, such as, forexample, percent or percent of the critical peak power, in order tosafely avoid increasing the amount of the total ore grinding mediacharge in the mill beyond that requiring the critical peak power draft.In accordance with the apparatus and method of the invention, theoperating set point of the power draft may be set at, for example,percent of the critical peak power, thereby increasing the grindingcapacity of the given grinding mill.

In describing the embodiment of the apparatus and method of theinvention shown in FIG. 1 a switching device has been shown whichselectively connects either the normal feed control 36 or the overridefeed control 52 in controlling relation to feed device 24. However, itwill be understood that other means may be employed to render either thecontrol 36 or the control 52 selectively either effective or ineffectiveto control the feed device 24. For example, instead of mechanicallyswitching either control 36 or control 52 into controlling relation todevice 24, the same result may be accomplished by maintaining acontinuous connection of both controls 36 and 52 to device 24, andselectively energizing or deenergizing the controls 36 and 52. For thisreason, the terms activate" and deactivate are respectively used in theclaims to cover any manner of respectively rendering effective (toactivate) or ineffective (to deactivate) the ability of control 36 or ofcontrol 52 to control feed device 24. Thus, for example, when control 36is disconnected from feed device 24, it has been deactivated and when itis connected to feed device 24 it is activated. In a similar manner, ifcontrol 36 were to be deenergized,

although connected to device 24, it would be deactivated and if it wereenergized while connected to device 24, it would be activated.

The terms activated and deactivated" are used in the same sense as justdescribed in the embodiment of FIG. 4, as well as in the embodiment ofFIG. 1.

All references herein to mesh sizes are to Tyler mesh sizes, a wellknown standard.

From the foregoing detailed description of the present invention, it hasbeen shown how the objects of the invention have been obtained in apreferred manner. However, modifications and equivalents of thedisclosed concepts such as readily occur to those skilled in the art areintended to be included within the scope of this invention.

I claim:

1. In combination, a grinding mill of the type which is capable ofoperating in the vicinity of its critical peak power draft, means forfeeding a grinding media to said mill, first control means for normallycontrolling the rate of feed of grinding media to said mill as afunction of the electrical power draft of the mill, second meansactuatable into overriding relation to said first control means atpredetermined intervals and effective when in said overriding relationto reduce the rate of feed of grinding media to said mill sufficientlyto cause a measurable effect on the power draft of the mill, means forsensing the direction of change in power draft of said mill caused bythe reduction in feed rate effected by said second means whereby toindicate where said mill is operating relative to the critical peakpower draft of the mill on the curve of power draft v. mill charge ofgrinding media for said mill, and means responsive (l) to a senseddecrease in the power draft of said mill caused by the reduction in feedrate effected by said second means for restoring said first controlmeans in controlling relation to the feed of grinding media to saidmill, and (2) to a sensed increase in the power draft of said millcaused by the reduction in feed rate effected by said second means formaintaining said second means in controlling relation to the feed ofgrinding media to said mill.

2. The combination defined in claim 1 in which said second means isoperative to substantially completely stop the feed of grinding media tosaid grinding mill.

3. The combination defined in claim 1 in which said grinding mill is asecondary autogenous grinding mill and the grinding media whose rate offeed is being controlled is the ore grinding media for said secondaryautogenous grinding mill.

4. The method of controlling a grinding mill of the type which iscapable of operating in the vicinity of its critical peak power draftwhich comprises the steps of: (1) Normally controlling the rate of feedof grinding media to said rnill by a normal feed control as a functionof the electrical power draft of said grinding mill; (2) overriding thenormal feed control at suitable intervals whereby to cause the rate offeed of the grinding media to decrease sufficiently to produce ameasurable effect on the power draft of the mill; and (3) sensing thedirection of change of the power draft of the mill caused by thedecrease in feed rate of grinding media when said normal feed control isoverridden as an indication of where the mill is operating relative tothe critical peak power draft of the mill on the curve of power draft v.mill charge of grinding media for said mill.

5. The method defined in claim 4 in which said grinding mill is asecondary autogenous grinding mill and the grinding media whose rate offeed is being controlled is the ore grinding media for said secondaryautogenous grinding mill.

6. The method defined in claim 4 in which the override action causes thefeed of grinding media to the mill to substantially stop whereby toproduce a measurable effect on the power draft of the mill.

7. The method defined in claim 4 which includes the step when a decreasein power draft by the mill is sensed in step (3) of reactivating thenormal feed control.

8. The method defined in claim 4 which includes the step when anincrease in power draft by the mill issensed in step (30f continuing toprovide a reduced rate of grinding media feed to said mill until adecrease in power draft by said mill is sensed.

2. The combination defined in claim 1 in which said second means isoperative to substantially completely stop the feed of grinding media tosaid grinding mill.
 3. The combination defined in claim 1 in which saidgrinding mill is a secondary autogenous grinding mill and the grindingmedia whose rate of feed is being controlled is the ore grinding mediafor said secondary autogenous grinding mill.
 4. The method ofcontrolling a grinding mill of the type which is capable of operating inthe vicinity of its critical peak power draft which comprises the stepsof: (1) Normally controlling the rate of feed of grinding media to saidmill by a normal feed control as a function of the electrical powerdraft of said grinding mill; (2) overriding the normal feed control atsuitable intervals whereby to cause the rate of feed of the grindingmedia to decrease sufficiently to produce a measurable effect on thepower draft of the mill; and (3) sensing the direction of change of thepower draft of the mill caused by the decrease in feed rate of grindingmedia when Said normal feed control is overridden as an indication ofwhere the mill is operating relative to the critical peak power draft ofthe mill on the curve of power draft v. mill charge of grinding mediafor said mill.
 5. The method defined in claim 4 in which said grindingmill is a secondary autogenous grinding mill and the grinding mediawhose rate of feed is being controlled is the ore grinding media forsaid secondary autogenous grinding mill.
 6. The method defined in claim4 in which the override action causes the feed of grinding media to themill to substantially stop whereby to produce a measurable effect on thepower draft of the mill.
 7. The method defined in claim 4 which includesthe step when a decrease in power draft by the mill is sensed in step(3) of reactivating the normal feed control.
 8. The method defined inclaim 4 which includes the step when an increase in power draft by themill is sensed in step (3of continuing to provide a reduced rate ofgrinding media feed to said mill until a decrease in power draft by saidmill is sensed.